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aChemical Engineering Department, Islamic

Tehran, IranbBiofuel Research Team (BRTeam), ABR

meisam_tab@yahoo.com; Fax: +98 26 3270

Cite this: RSC Adv., 2014, 4, 14352

Received 26th November 2013Accepted 10th March 2014

DOI: 10.1039/c3ra47033d

www.rsc.org/advances

14352 | RSC Adv., 2014, 4, 14352–1435

Experimental investigation of the effect of ceriumoxide nanoparticles as a combustion-improvingadditive on biodiesel oxidative stability: mechanism

Masoumeh Hajjari,a Mehdi Ardjmanda and Meisam Tabatabaei*b

Nano cerium oxide, a combustion-improving fuel additive, was

investigated for its impact on biodiesel oxidative stability. The findings

of the present study revealed for the first time that nano cerium oxide

addition at the concentrations generally used to improve combustion

(<100 ppm) severely reduced the oxidative stability of biodiesel.

1. Introduction

Biodiesel, an alternative to petroleum-derived diesel fuel (pet-rodiesel) is produced from vegetable oils, animal fats or usedfrying oils. Biodiesel has become widely acceptable in theenergy market owing to its unique features including reductionof most exhaust emissions in comparison with petrodiesel,higher cetane number, biodegradability, lack of sulphur,inherent lubricity, positive energy balance, higher ash point,compatibility with the existing fuel distribution infrastructure,renewability and domestic origin.1,2 Despite these advantageshowever, some signicant problems still exist from the tech-nical and commercial point of view i.e. higher NOx emissions,higher pour point and cloud points, and limited storage life dueto low oxidative stability.1,3 Among those, oxidative degradationof biodiesel during storage could severely compromise itsquality with respect to effects on kinematic viscosity, acid value,cetane number, total ester content, and formation of hydro-peroxides, soluble polymers, and other secondary products.4

This in turn could lead to decreased biodiesel stability andcould consequently jeopardize long-term and widespreadapplication of this green fuel.

Nitrogen oxide emissions – another disadvantage of bio-diesel – known as NOx are produced during the combustion ofbiodiesel and pose serious problems to both public health andthe environment.5 In light of that, a number of investigationshave aimed at addressing the mentioned drawbacks of

Azad University, South Tehran Branch,

II, 31535-1897 Karaj, Iran. E-mail:

1067; Tel: +98 26 32703536

6

biodiesel by addition of different additives e.g. metal basedadditives to biodiesel and its blends,5–7 On the other hand, newapproaches and advances achieved in nanotechnology haveencouraged researchers to take advantages of such metal-basedadditives in their nano-size.8 For instance, Sajith et al. (2010)and Hong (2011) mentioned that nanosized metal oxides (e.g.nano cerium oxide and nano aluminium oxide) exhibitedsignicantly higher activities and resulted in better impacts oncombustion characteristics of fuels when compared to theconventional additives.6,9 Moreover, another important advan-tage of nano-sized metal additives over their micron-sizedcounterparts would be the prevention of fuel injector and lterclogging.10 Such improvement could be ascribed to the fact thatat these dimensions the surface-area-to-volume ratio of theparticles increases considerably, leading to a larger contactsurface area and consequently more efficient performance.6,11

Among the additives studied nano cerium oxide has attracteda great deal of attention recently under various labels such ascombustion improver, engine performance improver, combus-tion-chamber surface cleaner,12 and NOx emission reducer. ArulMozhi Selvan et al. and Sajith et al.6,13 studied the effects ofcerium oxide nanoparticles on the engine performance andemission characteristics and physicochemical properties of bio-diesel and diesel biodiesel blends. They found a signicantreduction of harmful emissions, and improved brake thermalefficiency owing to the enhanced catalytic activity. Oxonica, a UK-based company also manufactured a fuel additive, Envirox,consisting of cerium oxide nanoparticulate, and reported fuelsavings of 10% when it was added to diesel fuel due to (a) bettercatalytic effect between diesel and air and (b) better oxygenabsorption and consequently reduced NOx emissions.14 In adifferent study, Jung et al. studied the kinetics of oxidativecombustion of biodiesel in the presence of cerium oxide nano-particles and revealed signicantly increased oxidation rate inthe combustion chamber.15 Despite all these promising attri-butes associated with the application of nano cerium oxide as acombustion enhancer, however, little is known about its impacton the stability of biodiesel and its shelf life.

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Table 2 Induction periods of biodiesel samples

Oxidation stability Biodiesel feedstock

Nano cerium oxide dosinglevels (ppm)

0 50 100 200 500

Induction period(IP)

Sunower oil 1.48 0.81 0.83 1.47 2.05Waste cooking oil 0.17 0.09 0.11 0.13 0.23

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Therefore, the present study was set to achieve an insightinto the potential positive or negative impacts of nano ceriumoxide on oxidative stability of biodiesel. To accomplish that,nano cerium oxide suspensions of neat biodiesel producedfrom sunower and waste cooking oils (SOB and WCOBrespectively) were examined using the Rancimat method(EN14112).

2. Experimental2.1. Materials

Pure sunower oil was purchased from a vegetable oil factoryand used frying oil was collected from a restaurant. Nanocerium oxide powder (99.97%) with 10–30 nm particle size waspurchased from US Research Nanomaterials, Inc. (USA)(http://www.us-nano.com).

2.2. Biodiesel synthesis and characterization

Biodiesel was produced from sunower oil and pre-treatedwaste cooking oil with methanol and an alkali catalyst (KOH) ina stirred tank reactor at 60 �C for 1 h. The pre-treatment processon waste cooking oil was carried out with the methanol-to-oilratio of 0.3 (v/v) in the presence of 1% H2SO4 (v/v) as an acidcatalyst for 1 hour at 60 �C. Aer the reaction, the mixture wasallowed to settle for 1 h and the methanol–water fractionseparated at the top was removed. The produced biodieselsamples were characterized based on the ASTM D6751 standardtest methods (Table 1).

2.3. Oxidation stability analysis

Nano cerium oxide was added to biodiesel samples at inclusionrates ranging from 0 to 500 ppm. The dosing levels of thecerium oxide nanoparticles required for each sample wasmeasured using a precision electronic balance apparatus andwere mixed with the fuel samples by means of a probe ultra-sonic instrument model MISONIX (40 kHz, 5 min). The oxida-tion stability of the samples were then investigated bydetermining the induction period as described by the EN14112standard test method using a modied Metrohm 743 Rancimatinstrument (Herisau, Switzerland). In this method, air (10L h�1) was sparged through the samples and then through awater trap. Volatile oxidation products (primarily formaldehydeand short-chain acids such as carboxylic acid) were absorbed bythe water, causing an increase in conductivity. The waterconductivity was monitored to determine the onset of oxidation

Table 1 Produced biodiesel specifications (limits are based on ASTMD6751 standard test method)

Property Unit SOB WCOB Limits

Density g cm�3 0.86 0.87 0.86–0.9040 �C viscosity mm2 s�1 3.9 5.2 1.9–6.0Flash point �C 178.3 170.2 >130Acid value mg KOH per g 0.15 0.6 <0.5Moisture % Volume 0.045 0.05 <0.05Iodine value g iodine per 100 g 58.5 109 120

This journal is © The Royal Society of Chemistry 2014

and to dene an induction time. Also, propyl gallate – aconventional antioxidant for biodiesel – was used as control(200–1000 ppm).

3. Results and discussion

Nano cerium oxide is being increasingly promoted as a bio-diesel-combustion improver and in particular NOx emissionreducer. However, as mentioned earlier its impact on one of thekey quality parameters of biodiesel i.e. stability has not yet wellinvestigated. The overall ndings of the present study obtainedthrough Rancimat analyses revealed that nano cerium oxideaddition (0–500 ppm), led to a signicant deterioration ofoxidative stability of both SOB and WCOB samples (Table 2).

As presented in Fig. 1, nano cerium oxide worsened theoxidative stability of both biodiesel samples at the lowestconcentration investigated (i.e. 50 ppm) but caused slightimprovements in comparison with neat biodiesel samples whenconcentrations over 200 ppm were used. Nonetheless, even atthe highest concentration of 500 ppm, nano cerium oxideaddition failed to meet the ASTM/EN requirement for biodieseloxidative stability (the induction period of 6 h). On the otherhand, application of nano-additives such as nano cerium oxideat concentrations over 100 ppm for improving fuel combustioncharacteristics is economically impractical.

Fig. 1 IP values determined using Rancimat test method for biodieselsamples dosed with 0–500 ppm nano cerium oxide.

RSC Adv., 2014, 4, 14352–14356 | 14353

Fig. 3 (a) Cerium oxide shift reaction between the two states ofcerium by oxygen releasing-absorbing mechanism. (b) Cerium oxiderole in combustion process: it absorbs oxygen from NO mediatesproduced due to the high temperature of combustion chamber, thendonates this oxygen to the soot (C) particles produced by incompletecombustion of hydrocarbons and converts them to CO2 molecules.

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Given the worsening effect of nano cerium oxide addition(within its concentration range as combustion improver,<100 ppm) on biodiesel oxidative features and in order to still beable to benet its combustion improving effects i.e. NOxreduction, the application a conventional antioxidant – propylgallate (PrG) (200–1000 ppm) – was also considered in thepresent study. The results obtained revealed that 500 and 1000ppm PrG were required to reach an induction period of above6 h, for sunower biodiesel and WCO biodiesel, respectively, inthe presence of 100 ppm nano cerium oxide. Whereas, signi-cantly lower concentrations of 200 and 800 ppm for neatsunower and WCO biodiesel samples, respectively, wererequired to meet the standard (Fig. 2).

3.1. Action mechanism of nano cerium oxide in biodiesel

As discussed earlier, nano cerium oxide is widely used as a fueladditive for the elimination of toxic exhaust emission gases. Infact, nano ceria can act as a chemically active component,working as an oxygen store by releasing oxygen in the presenceof reductive gases, and removing oxygen by interaction withoxidizing species.16 More specically, cerium oxide may absorboxygen for the reduction of NOx or may provide oxygen for theoxidation of CO and soot through the combustion6 (Fig. 3). Thekey to the use of ceria for catalytic purposes is the low redoxpotential between the Ce3+ and Ce4+ ions (1.7 V) allowing thebelow reaction to easily occur in exhaust gases.16 Therefore,both oxidative and reductive properties for nano cerium oxidecould be expected.

In case of the effect of nano cerium oxide on biodieselstability, based on the Rancimat results obtained, nano ceriumoxide's effect on biodiesel molecules did not follow a certainimproving or deteriorating trend. More precisely, despite thesharp reduction occurred in oxidative stability of the neat bio-diesel samples by the addition of 50–100 ppm of nano ceriumoxide, the stability increased slightly as the additive

Fig. 2 IP values determined using Rancimat test method for neat and100 ppm nano-additivated biodiesel samples versus the concentrationof propyl gallate.

14354 | RSC Adv., 2014, 4, 14352–14356

concentration increased to 500 ppm. In fact, nano cerium oxideseems to have mainly acted as an oxidizing agent for biodieselmolecules and facilitated peroxide radicals production bydonating oxygen species to the biodiesel radicals (Fig. 4), whileexerted some antioxidant capabilities when it was present athigh concentrations. It is worth quoting that anti-free radicaland antioxidant properties of this material have been previouslyobserved in biological systems such as DNA molecules, braincells, neurons, visual cells and lipid cells of living organ-isms.17,18 For instance, Estevez and Erlichman argued that nanoceria acts as a protective agent against oxidative cell damages bymoderately reducing the potent oxidizing agents in cells i.e.reactive oxygen species (ROS) accumulation.18

Cerium in nano cerium oxide lattice can reversibly bindoxygen and shi between Ce4+ and Ce3+ states under oxidizingand reducing conditions.19 The loss of oxygen and the reduc-tion of Ce4+ to Ce3+ are accompanied by the creation of oxygenvacancies in the nanoparticle lattice. Ce3+ lattice defects aswell as the resultant oxygen vacancies are abundant at thesurface of nano ceria.18,20,21 These oxygen vacancies can absorboxygen free radicals, neutralize them and then release oxygenmolecules to the media around them.20 It has been reportedthat the concentration of Ce3+ ions on the surface of theparticle increases by reducing nanoparticle size. Conse-quently, these would lead to increased anti-free radical abilityof ceria nanoparticles.22 Because of all these properties, severalstudies have proposed that nano ceria could act as free-radicalscavenger.21 These free radicals including peroxide andhydroxyl radicals could be produced during the degradation ofbiodiesel as well. However, based on the ndings of thepresent study, nano cerium oxide (in practical concentrations)did not seem capable of scavenging the large biodieselperoxide radicals.

To describe the different effects of nano cerium oxide in lowand high dosing levels a presumption could be postulated: atlow concentrations, cerium oxide lattice structure seems inca-pable of getting sufficiently close to the peroxide radicalsproduced during the rst step of biodiesel degradation toscavenge them. This is due to the high steric hindrance of largebiodiesel peroxide radicals. Under such conditions, only theoxygen donating property of cerium oxide could be observedwhereas at high concentrations of nano cerium oxide, theaccumulation of nanoparticle species around the peroxide

This journal is © The Royal Society of Chemistry 2014

Fig. 4 Nano cerium oxide oxidative-reductive acting mechanism on biodiesel molecules.

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radicals seems to overcome the steric hindrance. Then, thenanoparticles surround the peroxides produced in the propa-gation step of biodiesel oxidation chain reactions and preventtheir contact with the other biodiesel molecules. Thus, furtherdegradation of biodiesel molecules is inhibited by convertingthe free radicals to the former biodiesel molecules. Thisassumption is schematically shown in Fig. 4.

4. Conclusions

Nano cerium oxide is increasingly gaining attention in thebiodiesel industry owing to its combustion-improving features.The ndings of the present study revealed for the rst time thatnano cerium oxide (10–30 nm particle size) addition within theconcentration range ppm generally practiced to improvecombustion (<100), severely worsened the oxidative stability ofbiodiesel. An average 45% reduction in the IP values for bothbiodiesel samples was observed using 50 and 100 ppm nanoadditive. As a result, elevated concentrations of conventionalantioxidants such as PrG were required to make up for thisadverse effect while still enjoying nano cerium oxide benecialimpacts on biodiesel combustion. Considering the effects ofdecreasing nano cerium oxide particle size on its properties,such as increasing its free oxygen radical absorbance potential,using smaller particle sizes of this material may show betterimpacts on the stability of biodiesel, however, more research isrequired.

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